1. Introduction
This invention relates to copolymers of alkyl acrylates or methacrylates and dihydroxyalkyl acrylates or methacrylates which copolymers are particularly useful for the formation of contact lenses.
2. Description of the Prior Art
As is known in the art, the conventional contact lens is made from methyl methacrylate. Lenses made from this material, known as "hard lenses," have had only limited success because many people cannot adapt to the presence of the lens in the eye and/or the lens compromises the physiological processes required for corneal metabolism. For many people, minor irritations are caused by small particles and dust which become lodged under the lens and rub against the cornea. Moreover, it has been found that after wearing hard contact lenses for extended periods of time -- e.g., from 1 to 5 years, with varying degrees of success, many people develop discomfort and are forced to discontinue wearing the lenses.
In view of the above, there has been considerable research in an effort to develop new contact lens materials which would overcome some of the problems of the methyl methacrylate lens described above. A class of such materials is described in U.S. Pat. Nos. 2,976,576 and 3,220,960 incorporated herein by reference. The materials described in these patents are hydrogels of a sparingly cross-linked hydrophilic polymer and a substantial quantity of an aqueous liquid -- e.g. water. The hydropilic polymer is actually a copolymer of a major amount of a polymerizable monoester of an olefinic acid selected from the group of acrylic and methacrylic acids having a single olefinic double bond and a minor amount of a polymerizable diester of one of said acids, the diester having at least two olefinic double bonds. The copolymer is formed by copolymerization in a solvent medium.
Within the class of materials disclosed in these patents is a slightly cross-linked polymer comprising a predominant amount of 2-hydroxyethyl methacrylate. This polymer, known in the art as "Hema", has been primarily used for contact lenses. Hema has the significant characteristic of forming hydrophilic polymers capable of hydration with water -- e.g., typically about 40% by weight. (However, the percent may vary from approximately 35 percentage to 65 percent by weight). This high water content renders the contact lens made from the material quite flexible and soft, with the result that such a lens is able to mold itself to the curvatures of the eye more readily. This is in contrast to the conventional hard lens, which maintains a rigid manufactured configuration and does not conform to the eye's curvature.
The most significant advantage of the Hema hydrogel soft lens is that with proper design, it can be worn with almost immediate and continuing comfort as the cornea appears to suffer less discomfort than with the conventional hard lens at least initially. A second advantage of the Hema hydrogel soft lens is that relative to the hard lens, the problem of dust and foreign bodies becoming lodged beneath the lens and rubbing against the cornea is reduced. Still another advantage of the Hema hydrogel soft lens, as compared with the conventional hard lens, is improved peripheral vision resulting from the use of larger lens diameter.
Despite the advantages of the Hema hydrogel soft lens, there are still certain problems that have prevented, at least to date, the universal acceptance and use of such a lens. One such problem involves the lack of clarity of central vision. For many persons, the Hema hydrogel soft lens does not provide adequate and steady vision because the nature of the material results in a constantly changing optical surface during eye movement and blinking, possibly due to the lens being too soft or non-rigid. A second problem concerns the correction of astigmatism. Conventional hard contact lenses are usually able to correct corneal astigmatism by providing a new surface on the cornea. Because of the extreme flexibility of the Hema hydrogel soft contact lens, the lens will conform to the shape of the eye and therefore, in most cases, will not provide the new surface necessary to correct astigmatism. Other physiological problems have been found with existing Hema hydrogel soft contact lenses. These include corneal irritation and folds in the membranes of the eye. The exact causes or significance of these phenomena are not known, nor are there reported solutions. However, it has been reported that lacrimal interchange with Hema lenses is minimal as compared to conventional lenses, possibly because of the manner in which the lens conforms to the contour of the eye thereby preventing the flow of lacrimal fluid beneath the edge of the lens. The reduction in fresh lacrimal fluid is not desirable as it substantially reduces the contact of the eye with oxygen and relief from accumulation of catabolic products. Finally, the Hema hydrogel soft lens has been reported to have a tendency to tear easily, requiring replacement each time this occurs.
As will be discussed in greater detail below, there is provided herein a new hydrogel material suitable for the formation of contact lenses comprising a copolymer of hydrophilic monomers selected from the group of dihydroxyalkyl acrylate and methacrylate and a substantially water insoluble monomer selected from the group of an alkyl acrylate and a methacrylate, said hydrogel being formed by a bulk, free radical polymerization reaction. The preferred reactants are glyceryl methacrylate, and methyl methacrylate in certain specific proportions. Similar copolymers formed from the two preferred monomers are known in the art and described by H. Yasuada, C. E. Lamazo, and L. D. Ikenberry, Makromol. Chem. 118, 1935 (1968) and H. Yasuada, C. E. Lamaza and A. Peterline, J. Polym Sc. Part A-2, 996, 1117-1131 (1971).
The copolymers described in the first of the above two publications were polymerized in a 70/30 acetic acid/water solvent system. The solutions were prepared with 5% weight total monomer and 95% solvent, and initiated with approximately 0.5% (based on monomer weight) K.sub.2 S.sub.2 O.sub.8 and 1% Na.sub.2 S.sub.2 O.sub.2. Oxygen in the solutions was purged by bubbling nitrogen through the solution. After 8 to 10 days at room temperature, precipitation of the polymer was effected in water. Six copolymers were made in this series having ratios of methyl methacrylate and glyceryl methacrylate of from 95:5 to 70:30 (monomer mole ratio).
The procedure followed in the second of the above-noted publications was similar to the first except that the polymerization reaction was carried out in a non-aqueous solvent with 2,2' azobis (2 methyl propionitrile) as the initiator. For reasons to be discussed more fully below, the copolymers of these two references differ from those described herein as a result of the polymerization method used and the ratio of components, the compolymers of the references not having properties suitable for the purposes of this invention, especially with respect to the formation of contact lenses.